Conventionally, a color image processing apparatus such as a color copying machine, a color printer, and a color facsimile has been available. Such a color image processing apparatus reads a color original image by input means such as a color scanner, and outputs the copy image of the color original image by output means such as a color hard copying device.
Incidentally, in the above-mentioned color image processing apparatus, in the case of outputting inputted image data as they are by the output means, the inputted image data are almost always outputted as a copy image having different colors from those of the original image. This results from the fact that various types of input and output means differ in their characteristics.
Hence, color correction is carried out so as to faithfully reproduce the copy image having the colors of the original image.
As a common color correction technique, for example, "Color Correction Techniques for Correct Color Reproduction" in The Journal of Electrophotographic Institute, Vol. 29, No. 3 (1990) discloses the following color correction techniques: a color correcting process by encipherment using the polynomial regression analysis, the three-dimensional interpolation using a look-up table, and a color correcting process using a neural network without encipherment.
In the above-mentioned methods using the polynomial regression analysis, a color correction calculation is carried out using a color converting matrix. However, when a single color converting matrix is adapted for all the color regions, the accuracy is reduced. As a countermeasure, U.S. Pat. No. 5,552,904 discloses "Color Correction Method and Apparatus Using Adaptive Region Separation" wherein a color space is divided so as to carry out color correction with respect to an inputted image using a different color converting matrix for each divided region.
Namely, according to the color correction method disclosed in the above U.S. Patent, a color space is divided into a plurality of regions and a color converting matrix suited for each region thus divided is found, and the coefficient of the each color converting matrix thus determined is stored in a look-up table. The coefficient of the color converting matrix corresponding to inputted image data is selected so as to carry out color correction with respect to the inputted image data in accordance with the selected color converting matrix.
Note that, the above-mentioned U.S. Patent also teaches criteria for dividing the color space into a plurality of regions. However, as far as the color correction method is concerned, the same color correction methods but having different coefficients are adopted for both the divided color space and the color space which is further divided. Also, because the color space is divided into a plurality of regions, and a different color converting matrix is adopted for each region thus divided, the method disclosed in the described publication has a problem in that the continuity of color signals is not kept in the vicinity of the border line of each region.
Also, in a conventional color image processing apparatus adopting the color correction by the three-dimensional interpolation using a look-up table indicative of an input-output relationship, a problem such as an interpolation error is caused.
Further, in the case of extracting a certain region such as flesh tones to be accurately color-corrected so as to carry out the process of color correction on the region thus extracted, such a problem is caused wherein the continuity of color signals is broken in the vicinity of the border line of the color region thus extracted.